This description relates to buck-boost DC-DC switching power conversion, e.g., in converters for which the input operating voltage delivered to the converter may span a range of values extending below and above the magnitude of the DC voltage delivered at the output of the converter.
Electrical and electronic equipment and systems often require conversion of an input voltage to an output voltage, which may be higher or lower than or approximately the same as the input voltage. For example, in stationary or portable systems powered by a DC battery it is often necessary to maintain constancy of output voltages independently of the state of charge and voltage of the battery. As another example, in systems utilizing the new power distribution architecture described in Vinciarelli, U.S. patent application Ser. No. 10/006,481, “Factorized Power Architecture with Point of Load Sine Amplitude Converters”, filed Jan. 31, 2002 (the '481 application) and incorporated in its entirety by reference, it may be necessary to pre-regulate the “factorized bus voltage” delivered by a pre-regulator module (or “PRM”, as that term is used in the '481 application) to point-of-load voltage transformation modules (or “VTMs”, as that term is defined in the '481 application) by stepping up or down voltage from an input source. As another example, in systems powered from an AC voltage source it is often necessary to draw power from the AC source with near unity power factor while delivering power at a DC output voltage which may be higher or lower than the instantaneous voltage of the AC line. In general, it would be desirable to flexibly achieve the appropriate step up or step down of a voltage with high conversion efficiency, high power density, and low noise.
Buck-boost converters are known in the art. The buck-boost converter 10, shown in FIG. 1A, for example, is described in Severns and Bloom, “Modern DC-to-DC Switchmode Power Conversion Circuits,” 1985, ISBN 0-442-21396-4, pp. 156–157. In the converter of FIG. 1A the switches 2,3 are operated synchronously: switch 2 is in position “A” when switch 3 is in position “A” and vice versa. Energy delivered from the input source 6 is stored in inductor 4 when switch 2 and 3 are in the “A” position and energy is delivered from the inductor to the load 5 when switch 2 and 3 are in the “B” position. As also explained in Severns and Bloom, ibid., pp. 157–158, the converter of FIG. 1A may be reduced to a single switch buck-boost converter 12 of FIG. 1B, in which the output voltage, Vo, has a polarity inversion relative to the input source.
In both cases, owing to substantial losses in the inductor and switching elements, the converter architectures of FIG. 1A and FIG. 1B are inefficient relative to other architectures such as a buck converter or a boost converter which are only capable of, respectively, step-down or step-up of an input voltage. In fact, it has been tempting to conclude that the ability to provide both step-down and step-up of voltage within the same converter comes at a price in terms of reduced efficiency and power density.
This expectation has not been altered by more recent developments. A buck-boost converter incorporating four switches is described in an October 2001 datasheet for the LTC34401 “Micropower Synchronous Buck-Boost DC/DC Converter” integrated circuit manufactured by Linear Technology Corporation, Milpitas, Calif., USA. A simplified schematic of the converter circuit 14 is shown in FIG. 2. In the Figure the converter operates in a continuous conduction (i.e., the current in the inductor 23, IL is nonzero throughout the entire operating cycle). A switch controller 19 operates the four MOSFET switches 11, 13, 15, 17 in one of three modes: (1) in a buck mode, with switch 15 always closed and switch 17 always open, when the magnitude, Vin, of the input voltage source 6 is within a range of values which are greater than the voltage, Vo, delivered to the load; (2) in a boost mode, with switch 11 always closed and switch 13 always open, when the magnitude. Vin, of the input voltage source is within a range of values which are less the voltage, Vo. and (3) in a buck-boost mode, with a first pair of switches, 11 and 13, “phasing in” to achieve a minimum duty cycle for switch 13, as a second pair of switches, 15 and 17. “phases out” to reduce to zero the duty cycle of switch 17, as the magnitude, Vin. of the input voltage source traverses a range of values which bracket the value Vo. As such this “buck-boost” architecture merely bridges a transition from the boost architecture to the buck architecture, while incurring increased losses and an intermediate reduction of efficiency relative to boost and buck modes that it is bridging. Owing to continuous conduction in the inductor, in each of the three modes referenced above switching losses occur when certain switching elements are turned ON to carry current without the voltage across the switching element being reduced prior to turn ON. Even at light load where continuous conduction cannot be maintained with a finite value of inductor 23, a lossy damper circuit, comprising switch 16 and “anti-ring” resistor 18, is included to dissipate energy stored in the parasitic capacitances of the inductor and the switches.
Clamp circuitry for preventing oscillatory noise in switching power converters by using a switch to trap energy in an inductive element and release it to reduce switching losses is described in Vinciarelli et al, U.S. patent application Ser. No. 09/834,750, “Loss and Noise Reduction in Power Converters”, Apr. 13, 2001 (the “'750 application”), assigned to the same assignee as this application and incorporated in its entirety by reference.